1. Field of the Invention
The present invention relates to an endoscope apparatus, and particularly to a synchronous type endoscope apparatus for performing color image pickup.
2. Description of the Related Art
In recent years, electronic endoscopes including image pickup means have been widely employed in various endoscopic inspections and the like.
Electronic endoscope apparatuses used when performing endoscopic inspection include a synchronous type endoscope apparatus which performs color image pickup under white light as illumination light using an image pickup device provided with a color optical filter, and a frame-sequential type endoscope apparatus which performs image pickup respectively under frame-sequential RGB lights as illumination lights by using a monochrome image pickup device and generates a color image. The apparatuses have different image processing systems.
In addition, Japanese Patent Application Laid-Open Publication No. 2002-95635, for example, discloses an endoscope apparatus capable of displaying a running state of blood vessels in the vicinity of a mucosa surface layer with respect to the depth direction, which tends to be buried in optical information obtained under a normal visible light, as more easily identifiable image information by using a narrow-band illumination light.
In a synchronous type endoscope apparatus which performs color image pickup, a complementary color filter as shown in FIG. 18, for example, as a color separation filter 30 for optically separating colors, is mounted on an image pickup surface of a CCD as an image pickup device, on a pixel basis.
The complementary color filter has color chips in four colors, magenta (Mg), green (G), cyan (Cy) and yellow (Ye) which are arranged in front of the respective pixels with Mg and G being alternately arranged in the horizontal direction, and the arrays Mg, Cy, Mg, Ye and G, Ye, G, Cy being arranged in that order in the vertical direction.
In the CCD using the complementary color filter, two columns of pixels adjacent to each other in the vertical direction are added and sequentially read out such that the columns of pixels are shifted with respect to each other in odd fields and even fields. A luminance signal Y and color difference signals Cr, Cb are then generated, as is known, by the color separation circuit in a subsequent stage.
Specifically, when reading out the pixels in n-line, readout signals are “Mg+Cy”, “G+Ye”, . . . and when reading out the pixels in n−1 line, the read signals are “Mg+Ye”, “G+Cy”, FIG. 19 shows an example of spectral sensitivity characteristics of these readout signals “Mg+Cy”, “G+Ye”, “Mg+Ye” and “G+Cy”.
Then, the luminance signal Y and the color difference signals Cr, Cb having the spectral characteristics as shown in FIG. 20 are generated by the color separation circuit in the subsequent stage, and the signals passes a known matrix circuit to be converted into RGB signals.
However, the spectral sensitivity characteristics of each of the CCDs having the complementary color filter sometimes differ from each other. FIG. 21 shows the spectral sensitivity characteristics of readout signals of a CCD (for example, a second CCD) which is different from the one shown in FIG. 19.
When the spectral sensitivity characteristics of the readout signals thus differ, the spectral sensitivity characteristic of the RGB signals obtained through the matrix circuit also differs from the first CCD to the second CCD, for example, as shown in FIGS. 23, 24.
Actual image signal intensity reflected on the image is obtained based on the spectral sensitivity characteristics of the respective RGB signals. That is, when the intensity of a signal X (X is one of R, G, and B) of a pixel (i, j) is assumed to be X(i, j), the actual image signal intensity can be obtained by using the following expression.[Expression 1]X(i,j)=∫E(λ)·Sx(λ)·O(i,j,λ)dλ  (1)
Here, E(λ) represents a comprehensive spectral product obtained by multiplying the spectral radiance of the light source, the spectral transmission factors of the infrared cut filter and the condensing lens, the spectral transmission factor of the light guide of the endoscope, the spectral transmission factors of the illumination lens and the objective lens provided in front of the CCD, and the like. Sx(λ) represents the spectral sensitivity of the signal X (X is one of the R, G, and B) calculated through the matrix circuit based on the spectral sensitivity of the CCD. O (i, j, λ) represents the spectral reflectance of the subject.
Therefore, even if the spectral product E(λ) and the spectral reflectance of the subject O(i, j, λ) are the same, when the spectral sensitivities Sx(λ) of the RGB signals change due to the difference in the spectral sensitivities of the CCDs mounted to the endoscope, the intensity balance of the RGB signals changes when observing a living mucosa whose spectral reflectance complexly changes in integral wavelength ranges though white balance processing is performed in a white balance circuit. As a result, image quality differs in the reproduced color tone from one endoscope to another.